1. Field of the invention
The present invention relates to a power line connection circuit for controlling the connection of an electrical device to a power supply line and a switching IC for a power supply line. More specifically, the invention relates to a power line connection circuit which has a small turn-on resistance and has a discharge circuit in order to stabilize a voltage of the power supply line, the discharge circuit quickly discharging the electric charge in a capacitive element when the power supply line is disconnected and does not permit a current to flow into the discharge circuit when the power supply line is connected.
2. Description of the Related Art
In electrical devices, it is important to reduce power consumpution. Particularly, when a battery is used as a power supply, power consumption is very important because power consumption influences the life of the battery. Namely, power consumption determines the time for which the battery can be used without being exchanged or charged. There has been proposed a power conservation method in which a device is divided into a plurality of portions, and the supply of power to non-operating portions is shut-off. Many kinds of electrical devices include functional portions which must always operate even if the devices are not being used. For example, in an electrical device having a timer function, the timer function must operate even when the device is not being used. However, if power is supplied to the whole device in order to operate the timer function even when the device is not used, large amounts power are consumed because standby currents flow in the portions which are not needed. Therefore, when the above power conservation method is applied to this device, the device is divided into a main portion and a sub-portion having a timer function. Power is not supplied to the main portion when the device is not used but power is supplied to the sub-portion even when the device is not used.
Further, in a computer system, a plurality of peripheral devices such as a display, a disk unit, a printer, etc. are connected to a computer. These peripheral devices may not always operate even when the computer operates. Therefore, power consumption of the computer system can be reduced by stopping the supply of power to peripheral devices which are not needed.
In order to perform the above method, it is necessary to freely stop the supply of power to each portion. A power line switch IC is used to control connection between the power supply line and each portion. The switch IC is controlled by a control signal output from a control portion included in the sub-portion which always operates.
In general, a power FET has been used as the power line switch, and the power line connection circuit has heretofore been realized as a discrete circuit using the power FETs.
In the power line connection circuit, it is important that the resistance of the switch is so small that it can be neglected when it is turned on and that the switch is completely cut off when it is turned off. When the switch has a resistance when it is turned on, the voltage drops correspondingly and electric power is wasted. When the switch is not completely cut off when it is turned off, a current flows in trace amounts into the device unit when the switch is turned off and the electric power is consumed. In the conventional power line connection circuit using the MOS transistor, therefore, the MOS transistor must operates such that the turn-on resistance is very small, and the turn-off resistance is very large.
For instance, assuming that the voltage of the power supply line is 5V, an N-channel MOS transistor has a very large resistance when 0V is applied to its gate, and it has a very small turn-on resistance when a voltage higher than 5V is applied to its gate. (In the following, the turn-on resistance is called ON resistance, and the turn-off resistance is called OFF resistance.) Therefore, in the conventional power line connection circuit using an N-channel MOS transistor, a voltage higher than a positive logical value of the control unit is generated, and this voltage is applied to the gate of N-channel MOS transistor. In the case of the P-channel MOS transistor, a voltage higher than a positive logical value is applied to the gate of P-channel MOS transistor for obtaining a very large OFF resistance.
Therefore, in the conventional device including the power line connection circuit, a voltage conversion circuit is provided in order to generate the voltage higher than a positive logical value of the control unit.
Further, in some kinds of electric devices, supply voltages to portions are changed between different voltages according to operation modes. For example, in an electrically erasable nonvolatile semiconductor memory which is called a flash-memory, only 5V is supplied for reading operation, and 12V is supplied for writing and erasing operations. When N-channel FETs having same characteristics are used as switches for the 5V and 12V power supply lines, the ON resistances of these switches are different because an ON resistance of an N-channel FET changes according to the voltage between the gate and the drain of the FET.
Electric devices generally have specifications in which acceptable ranges of ON resistances of power supply lines are determined. When N-channel FETs having the same characteristics are used in power line connection circuits of different voltages, a problem may result in that a few connection circuits cannot satisfy the above ON resistance specifications. In those cases, the specifications have been satisfied by using N-channel FETs having different characteristics, or by changing the output voltages of the voltage conversion circuits. For example, the output voltage can be adjusted by resistances of resistors used in the voltage conversion circuits. However, it is not efficient for production processes to use different N-channel MOS transistors or to adjust elements. These problems cause production cost increases.
When different power supply voltages are respectively supplied to different portions and power line connection circuits of these portions are realized by using similar FETs, the same problem again ocurrs.
As described above, the power line connection circuit has heretofore been realized by a discrete circuit using a power FET, and a voltage conversion circuit which generates a voltage applied to the gate of the power FET. However, this results in high production costs. It has been desired to reduce the size of the circuit and the cost of production by realizing the power line switches in the form of ICs.
Further, because the voltage of a control signal applied to a gate of the power FET is constant, there occurs a problem that ON resistances cannot satisfy the specifications when power FETs having same characteristics are used for different power supply voltages. Particularly, when a plurality of power FETs are integrated in an IC, it is impossible to respectively adjust the ON resistances of the power FETs. Therefore, it is difficult to make the power line connection circuits of different voltages by using an IC which includes several power FETs.
A capacitive element is usually connected to stabilize the voltage of the power supply line. By using such a capacitive element, the power supply voltage is stably maintained against the fluctuation in the electric power consumed by the device unit. However, when the capacitive element is connected, the voltage of the power supply line gradually decreases when the switch for power supply lines is turned off due to the electric charge stored in the capacitive element, therefore, there arises a problem of an increase in the erroneous operation in the device unit. In a conventional device, a discharge circuit in which a resistor element is connected in parallel with a capacitive element is provided in order to prevent the occurrence of erroneous operation.
In the constitution having a discharge resistor, furthermore, there exists a problem in that a current flows at all times from the power supply line to ground via the resistor when the power line switch is turned-on. The resistor generally has a very large resistance, and the consumption of electric power due to the current flowing through the resistor is very small. However, such a consumption of electric power could become a serious problem in the devices which use batteries.